Defoaming agents having a silicon base are known to consist of alkyl or aryl-substituted polysiloxanes and colloidal silicic acid. The effectiveness of these defoamers depends mainly on their stability in the solution to be defoamed and thus on their pretreatment.
A method for the preparation of stable polysiloxane defoaming agents in which the powdered silicic acid to be used is first "silanated" with low molecular siloxanes, such as octamethylcyclotetrasiloxane or tetradecamethylhexasiloxanes, by heating to temperatures above 350.degree. C., and then suspended in dimethylpolysiloxane oil and heated at temperatures of approximately 200.degree. C. for several hours, is known from U.S. Pat. No. 3,113,930. The defoaming agents obtained are stable in alkaline solutions. According to U.S. Pat. No. 3,235,509, Lewis acids are added as condensation catalysts during the heating of methylsiloxane polymer, which vitiates pretreatment of silicic acid to render it hydrophobic. The use of alkaline catalysts, for example, alkali metal or tetraalkylammonium hydroxides, for that purpose, is recommended in U.S. Pat. No. 3,560,401. The catalyst acts only at the interface between polysiloxane and filler in this method, and cleaving as well as polymerization reactions do not occur.
A different approach is disclosed in U.S. Pat. No. 3,455,839. According to this reference, mixtures of (1) dimethylpolysiloxanes with a viscosity of 20 to 1,500 cs. at 25.degree. C., (2) resin-like organopolysiloxanes built up of (CH.sub.3).sub.3 SiO.sub.1/2 and SiO.sub.2 units at a ratio of 0.6:1 to 1.2:1, and (3) silicic acid aerogels are prepared for the preparation of stable demulgators by stirring the individual components together in random sequence at room temperature or at temperatures up to 170.degree. C. Catalysts are not used, so that only a fixation of the silicic acid, but not a cross-linkage reaction between the dimethylpolysiloxane and the resin-like polysiloxane, takes place. Consequently the products of the process consist mainly of mixtures and not of block polymers.
The foam inhibitors described are effective in aqueous alkali or tenside solutions and are sufficiently stable that no loss of foam inhibiting effect occurs during the period of application, for example, during a washing and cleaning process. However, when the inhibitors are mixed with washing agents of conventional composition, which contain alkalies and tensides, for example, their effectiveness is somewhat reduced with extended storage time, so that foam problems can occur with the use of these agents after a storage period of several weeks. This loss of effectiveness is even greater when the foam inhibitors are mixed with the paste-like washing slurry, in the interest of a homogeneous distribution in the washing agent, and, together with the other constituents of the washing agent, spray dried.
For the prevention of a detrimental interaction of the polysiloxane demulgator with the alkaline components of a cleaning agent, U.S. Pat. No. 3,829,386 recommends, for example, the emulsifying of the polysiloxanes in an aqueous solution of a non-ionic tenside, and the addition of this emulsion to the products containing alkalies. According to U.S. Pat. No. 3,843,558, the organopolysiloxanes are first added to sodium tripolyphosphate, and this premix is then combined with the remaining washing agent components. U.S. Pat. No. 3,933,672 discloses that the interactions between the foam inhibitors and the tensides contained in washing and cleaning agents are held responsible for the reduction in foam inhibiting activity, and it is recommended that the polysiloxanes be added to a vehicle and also that the granulates obtained be covered with a coating material that cannot be penetrated by tensides. Among the polysiloxanes recommended for this method are, preferably, mixtures of polydimethylsiloxanes, siloxane resins, and silanted silicic acid, which thus correspond to the products of the process according to U.S. Pat. No. 3,455,839.
The disadvantage of the described methods or products of the methods is seen in the fact that they cannot be distributed homogeneously in the washing agent by a simple method, for example, by introduction into the washing agent slurry and subsequent spray-drying. On the contrary, multi-step granulating and mixing processes are needed to prevent a reduction of the effectiveness of the foam inhibitors during preparation and storage of the products. Particular problems arise with respect to the last mixing step, that is, the homogeneous incorporation of the relatively small amount of powder premix containing siloxane into the main bulk of the washing agent. An intensive mixing or milling necessarily would result in poorer powder quality due to formation of dust and destruction of the hollow sphere structure of the spray-dried powder components, while, on the other hand, inhomogeneities can cause foaming problems during use.